Method and apparatus for controlling the directivity pattern of an antenna

ABSTRACT

The present disclosure relates to a method and apparatus for varying the directional characteristics of a nonresonant antenna system by varying the impedance of the antenna terminating impedance and, more especially, to making the terminating impedance different from the characteristic impedance of the antenna. Means for interchanging the roles of the &#39;&#39;&#39;&#39;feed&#39;&#39;&#39;&#39; and &#39;&#39;&#39;&#39;load&#39;&#39;&#39;&#39; ends of the antenna in order to provide a directivity pattern and its mirror image directivity pattern and means for utilizing this pattern and its mirror image to precisely locate the direction to a transmitting source are also disclosed.

United States Patent [191 West [ June 18, 1974 METHOD AND APPARATUS FOR CONTROLLING THE DIRECTIVITY PATTERN OF AN ANTENNA Raymond H. West, Fort Wayne, Ind.

[73] Assignee: The Magnavox Company, Fort Wayne, Ind.

22 Filed: July 12, 1971 211 App]. No.: 161,811

[75] Inventor:

[52] [1.8. CI 343/120, 343/114, 343/731, 343/749, 343/845, 343/861 [51] Int. Cl. G0ls 3/20 [58] Field of Search 343/120, 739, 740, 731, 343/738, 114

[56] References Cited UNITED STATES PATENTS 2,968,035 1/1961 Sirons 343/120 3,118,143 1/1964 Burns 343/120 3,596,272 7/1971 Blonder 343/740 X Primary Examiner-Samuel Feinberg Assistant Examiner-Richard E. Berger Attorney, Agent, or Firm-Roger M. Rickert; Richard T. Seeger [5 7] ABSTRACT The present disclosure relates to a method and apparatus for varying the directional characteristics of a nonresonant antenna system by varying the impedance of the antenna terminating impedance and, more especially, to making the terminating impedance different from the characteristic impedance of the antenna. Means for interchanging the roles of the feed and load ends of the antenna in order to provide a directivity pattern and its mirror image directivity pattern and means for utilizing this pattern and its mirror image to precisely locate the direction to a transmitting source are also disclosed.

12 Claims, 7 Drawing Figures SYNCHRONOUS COMPARATOR 'ND'CATOR 55 PATENTEU a18',48o

SHEET 1 0F 4 IMPEDANCE 47 MATCH DETECTOR -49 SYNCHRONOUS COMPARATOR INDICATOR 55 INVENTOR RAYMOND H. WEST BY MW ATTO R N EYS Pmmsnmw 3.818.480

SHEEI20F4 w W M M M WMWMT WWW Wm M MT! F" -T GT2 if f q A INVEN TOR RAYMOND H. WEST ATTORNEYS l2 MATCHED I TERMINATION MISMATCHED 6 TERMINATION DIFFERENCE (dB) w ANGLE FROM BORESIGHT (DEGREEs) RESOLUTION IMPROVEMENT AT BORESIGHT RECEIVER IMPEDANCE IMPEDANCE RECEIVER I l G? 5 INVENTOR RAYMOND H. WEST BY WWW ATTORNEYS DIFFERENCE (d8) PATENTED T 3 1974 SHEET '4 BF 4 RL= 0.6 2 RL= 0.4 2

R O. I 20 ANGLE FROM BORESIGHT (DEGREEs)-- INVENTOR RAYMON D H. WEST ATTORNEYS METHOD AND APPARATUS FOR CONTROLLING THE DIRECTIVITY PATTERN OF AN ANTENNA BACKGROUND OF THE INVENTION This invention relates to direction finding or homing antennas and more especially to such antennas of the terminated transmission line type. Such terminated transmission line antennas, also sometimes referred to as loaded loop antennas in an equivalent form, are well known in the art and all terminate in impedances matched to the antenna transmission line characteristic impedance. For short antennas of this variety, for example, having a length less than one-fourth wave length, the matched termination impedance ideally results in a cardioidal-shaped directivity pattern. Two such antennas 180 out of phase with each other may be provided, and if the two are properly matched, equal signals will indicate the appropriate source direction.

Such known antenna systems are generally constructed and tested in conjunction with an ideal or nearly ideal ground plane, and their directivity pattern may suffer some distortion when they are installed on a nonideal ground plane such, for example, as an aircraft. Nearby obstructions may also vary their directivity pattern. Further, such antennas have no provision for control over their angular resolution characteristics in a given application.

SUMMARY OF THE INVENTION The present invention overcomes the two aforementioned drawbacks of prior art nonresonant antennas by providing a mismatched variable terminating impedance which may be used both to correct the directivity pattern for vagaries introduced due for example, to mounting the antenna on a nonideal ground plane and to vary the antenna pattern to improve the angular resolution characteristics in a given environment. Such a mismatch makes the antenna pattern somewhat frequency dependent while substantially improving the angular resolution for direction finding purposes. In accomplishing these results according to the present invention, a single transmission line antenna terminated in a mismatched impedance is used in conjunction with a switching circuit for periodically interchanging the ends of the antenna to which the load and receiver, transmitter or other antenna utilization means are connected. This switching circuitry comprises a pair of semiconductor devices connected in series between the ends of the antenna with their junction coupled to the antenna utilization device, each semiconductor being alternately biased to states of conduction and nonconduction. Two further semiconductor devices also alternately biased between states' of conduction and nonconduction are connected to two variable terminating impedances and all four semiconductor devices are properly biased at the appropriate time by a single square wave generator.

Accordingly, it is one object of the present invention to provide a nonresonant antenna system which may be appropriately adjusted after installation on a nonideal ground plane.

It is another object of the present invention to provide a nonresonant antenna system the directivity pattern of which may be easily varied.

It is a further object of the present invention to provide a nonresonant antenna system easily switchable to provide the illusion of a symmetrically mounted pair of nonresonant antennas and which may be appropriately adjusted for its given environment.

Yet another object of the present invention is to provide a scheme for varying the directivity pattern of a nonresonant antenna.

A salient object of the present invention is to provide a direction-finding antenna, the pattern of which may be varied to improve resolution.

These and other objects and advantages of the present invention will appear more clearly from the following detailed disclosure of a preferred embodiment read in conjunction with the accompanying drawing in which:

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic diagram of the antenna system of the present invention;

FIG; 2 illustrates wave forms at the similarly marked points of FIG. 1 under varying input conditions;

FIG. 3 illustrates the cardioidal shaped sensitivity pattern of the system of FIG. 1 when the antenna is terminated in its characteristic impedance;

FIG. 4 is a graph illustrating the improvement in resolution achievable by the present invention;

FIG. 5 is a simplified form of an alternative version of the invention;

FIG. 6 shows a pattern analogous to FIG. 3 for several mismatched termination impedances; and

FIG. 7 is a graph analogous to FIG. 4 for the several mismatches illustratedin FIG. 6.

DESCRIPTION OF THE PREFERRED EMBODIMENT Turning first to FIG. 1, a nonresonant antenna 11 is insulatingly mounted on a ground plane 13, and the first end 15 of the antenna 11 is coupled by way of a coaxial cable 17 and a capacitor 19 to a first junction 21 between two diodes 23 and 25. The cathode of diode 25 in turn is coupled by way of a variable impedance circuit 27 to ground or another point of reference potential.

The circuit as thus far described has striking symmetry, thus, for example, there are a total of four diodes connected in series adjacent ones of which are coupled anode to cathode. Diodes 29 and 31 define a second junction 33 which is coupled by way of capacitor 35 and coaxial cable 37 to the other end 39 of the antenna 11. The anode of diode 31 is similarly connected to ground by way of another variable impedance network 41.

The junction between the two centrally located diodes 23 and 29 is connected to ground by way of a radio frequency choke 43 and also coupled, for example, by another coaxial cable 45, to an antenna utilization device. The antenna utilization device could, of course, be any type of transmitter or receiver; however, in the present preferred embodiment, the antenna system is to be used as a direction finding or homing device and, accordingly, the antenna utilization device consists of an impedance matching network 47 which in turn is coupled to a detector 49 which serves to demodulate the received radio frequency signal. The detector 49 may be a simple diode which would provide the wave forms illustrated in FIG. 2 or may be any type of receiver'and might only provide the envelope of the relevant curves of FIG. 2 as output to the synchronous comparator 51. As will become more apparent subsequently, the output of this detector is a level modulated square wave, the level or amplitude of which is dependent upon the relative orientation of the antenna 11 with respect to the source of the radio frequency signal. While we will assume an unmodulated carrier wave input for the purpose of explanation, the system works equally well for a modulated carrier so long as the modulation does not correspond precisely to the signal from the source 53. The relative orientation of the signal source and the antenna then may be indicated by readings on a level indicator 55. The first and second junctions between the first and second and the third and fourth diodes, respectively, are coupled by way of radio frequency chokes 20 and 36 to a source of a square wave signal 53. This square wave signal source 53 and the square wave output from the detector may be appropriately combined in a synchronous comparator 51 to resolve part of the directional ambiguities inherent in switched antenna type direction finding equipments.

Some of the principles and features of the present invention may be more easily understood from FIG. which illustrates an alternative form of the present invention in simplified form. In FIG. 5, the antenna 11 is illustrated as a transmission line which is according to well-known principles equivalent to the antenna 11 illustrated in FIG. 1 when combined with its ground plane 13. A ganged switch S1, S2 serves to connect a receiver 57 to one end of the antenna and a terminating impedance 59 to the other end of the antenna when the switch is in one position, and when the switch is reversed to the position shown, the terminating impedance 61 is now connected to the first end of the antenna and a receiver 63 is connected to the other end of the antenna. Operation of the switch S1, S2 serves to effectively reverse the sensitivity pattern of the antenna 11' and performs the same function as the square wave generator 53 along with the several diodes and associated circuitry illustrated in FIG. 1.

Assume for the moment that the switch S1, S2 is in the position shown in FIG. 5 and that the terminating impedance 61 (along with any capacitors, coaxial cables, etc., which may serve to connect the actual impedance structure to the end of the antenna) is equal to the characteristic impedance of the antenna. This assumption is characteristic of all prior art attempts and represents a matched load for the nonresonant antenna. Under these circumstances, the antenna 11 has the directivity pattern illustrated in FIG. 3 as a cardioid if the length of the antenna is small, for example, in terms of one-fourth wave length. If the position of the switch S1, S2 is now changed, the cardioidal pattern illustrated as a dotted line in FIG. 3 represents the directivity pattern of the antenna 11'. As is well-known the direction of maximum sensitivity lies along the antenna and in the direction toward the load impedance. Thus, rapidly reversing the switch gives the illusion of having two antennas 180 out of phase with each other.

Considering FIGS. 3 and 5 together, assume that an incoming signal is directed along the vector A. Under these circumstances, the receiver 63 will give a maximum output signal and the receiver 57 will have no output signal. If the incoming signal is directed along vector B, receiver 63 will have a relatively large output signal and receiver 57 a relatively small output signal. An incoming signal along vector C will give equal output signals from each of the receivers assuming, of course,

that the receivers are a substantially matched pair. Thus, it is clear that direction finding may be easily accomplished by determining when the signals from receivers 57 and 63 are equal.

A signal incoming along vector D will provide relatively large output from receiver 57 and a relatively small output from receiver 63; however, if as illustrated the vectors B and D are symmetrically disposed about vector C, a knowledge of which receiver is receiving the larger signal is necessary in order to determine whether this signal is incoming along vector B or along vector D. Further ambiguity inherent in the present as well as the prior art antennas involves distinguishing between signals incoming along vector B and along vector E out of phase from that along vector B. Recourse to prior art procedures such as the use of an omnidirectional antenna in conjunction with the present antenna would resolve these 180 ambiguities. The ambiguities between vectors symmetrically disposed about the null axis or boresight (vector C) are resolved by way of a phase comparison effected by the synchronous detector or comparator 51 of FIG. 1 since the circuit of FIG. 1 has but a single receiver.

Returning now to the preferred embodiment of FIG. 1, the first end 15 of the antenna 11 is connected to the receiver (detector 49) by way of coaxial cable 17, capacitor 19, diode 23, coaxial cable 45 and the impe dance matching circuit 47 during the negative half of the cycle of the square wave oscillator since the square wave oscillator causes the two junction points 21 and 33 to be negative relative to ground during the negative portion of its cycle. If the junction point 21 is negative, diode 25 is biased to a nonconducting state and diode 23 is biased to a conducting state, it being connected to a point of higher potential (ground) by way of radio frequency choke 43 which presents a very small impedance at the frequency of the square wave generator. The amplitude of the square wave generator signal should, of course, be sufficiently large to insure that diode 23 is conducting for all portions of the incoming radio frequency signal unless it is desired to effect the detection by a more sophisticated biasing of the diodes 23 and 29. Also during the negative portion of the square wave cycle, the diode 31 is biased to its conducting state, and the diode 29 is biased to its nonconducting state. Under the circumstances, the other end 39 of the antenna 11 is electrically connected to the variable impedance circuit 41 by way of the coaxial cable 37, the capacitor 35 and diode 31. During the positive half of each cycle of the square wave generated by the square wave generator 53, diode 25 is in its conducting state, diode 23 is nonconducting, diode 29 is conducting and diode 31 is nonconducting. Thus, during the positive half cycle, the second end 37 of the antenna is coupled by way of coaxial cable 37, capacitor 35, diode 29, coaxial cable 45 and impedance matching circuit 47 to the detector 49 while the first end 15 of the antenna is coupled by way of coaxial cable 17, capacitor 19 and diode 25 to the variable impedance circuit 27. Also present in the circuit of FIG. 1 are a pair of capacitors 19 and 35 which present negligible impedance to the incoming radio frequency signal yet present substantial impedance to the low frequency square wave output of the square wave generator 53 and a pair of radio frequency chokes 20 and 36 which present negligible impedance to the square wave signal yet substantial impedance to the incoming radio frequency sigstantially different (and easily detectable as illustrated in P10. 7) due to well defined sharp cusps occurring along the axis. This is due to a greater slope of the directivity pattern where it intersects the boresight. It is precisely this sort of sharp cusp (sometimes a null) which is desired and which allows accurate direction finding.

Thus, while the present invention has been described with respect to a specific preferred embodiment, numerous modifications will suggest themselves to those of ordinary skill in the art, for example, antennas of types other than those disclosed may be terminated in a noncharacteristic impedance and such mismatched terminated antennas may be used for other than direc tion finding or homing purposes. Accordingly, the scope of the present invention is to be measured only by that of the appended claims.

1 claim:

1. A direction finder antenna system having controllable directional characteristics comprising:

an antenna having a characteristic impedance and having first and second ends;

a load adapted to function as an antenna terminating impedance and having an impedance different from the said characteristic impedance, the antenna system having different directional characteristics for different impedance values of said load;

antenna utilization means; and

means connecting said antenna utilization means to one of the first and second ends of the antenna and said load to the other of the first and second ends of the antenna.

2. The antenna system of claim 1 further comprising a second load adapted to function as an antenna terminating impedance and having an impedance different from the said characteristic impedance, the antenna system having different directional characteristics for different values of said load, and means for periodically interchanging the ends of said antenna to which said utilization means is connected and for connecting one of said loads to the end of said antenna opposite the utilization means.

3. The antenna system of claim 2 further comprising means for independently varying the impedance of each said load to vary the directional characteristics of the antenna system.

4. An antenna system comprising:

an antenna having a characteristic impedance and having first and second ends;

antenna utilization means;

first and second loads each having a variable impedance different from the said characteristic impedance to thereby permit the directional characteristics of the antenna to be varied;

a square wave generator having a frequency substantially less than the frequency to be received by the antenna system;

first, second, third and forth semi-conductor devices, the first and second semi-conductor devices being connected in series between the antenna ends and having their common point connected to said utilization means, the third and forth semi-conductor devices each connected to a respective one of the antenna ends and to a respective one of the first and second loads and adapted to be switchably biased by the said square wave generator one to a conducting state and the other to a non-conducting state whereby one said variable impedance means is coupled by way of the third semi-conductor device to one of said antenna ends and the utilization means is coupled by way of the second semiconductor device to the other of said antenna ends when said second and third semi-conductors are biased to their conducting state by the square wave generator and the other of said variable impedance means is coupled to the other antenna end by way of the forth semi-conductor device and the utilization means coupled by way of the first semiconductor device to said one antenna end when the first and forth semi-conductor devices are biased to their conducting state by the square wave generator.

5. A symmetrically switchable variable impedance antenna loading circuit comprising:

a series circuit of four diodes adjacent ones of said diodes being coupled anode to cathode;

an antenna having a first end connected to a first junction between the first and second diodes and a second end connected to a second junction between the third and fourth diodes;

at least two variable impedance means, one coupled between the cathode of the first diode and a point of reference potential and the other coupled between the anode of the fourth diode and said point of reference potential;

means coupling a third junction between the second and third diodes to an antenna utilization device; and

means for periodically varying the potential at said first and second junctions.

6. The antenna loading circuit of claim 5 wherein said means for periodically varying comprises a square wave oscillator having an output frequency substantially less than the frequency to be received by said antenna; and

radio frequency choke means coupling said square wave oscillator to said first and second junctions, the impedance of said radio frequency choke means being relatively high at the frequencies to be received by said antenna and relatively low at said square wave oscillator frequency.

7. The method of loading a nonresonant antenna system for at least one of; compensating for irregularities introduced by mounting said antenna on for example an irregular ground plane, and varying the directional characteristics of said antenna system, said method comprising the steps of:

connecting one end of an antenna to a utilization device; and

connecting the other end of said antenna to a load having an impedance different from the characteristic impedance of said antenna.

8. The method of claim 7 further comprising the step of varying the impedance of said load.

9. The method of claim 8 further comprising the step of periodically interchanging the ends of said antenna to which said utilization device and the load are connected.

10. The method of claim 7 further comprising the step of periodically interchanging the ends of said antenna to which the load and said utilization device are connected.

nal. These last mentioned chokes and capacitors, simply stated, serve to prevent the antenna 1 1 from effectively bypassing the diodes 23 and 29 so far as the square wave signal is concerned and prevent the line 50 from effectively shorting out the radio frequency input.

The two variable impedance circuits 27 and 41 may be any type of impedance; however, in one preferred embodiment, these two circuits each consisted of the parallel combination of a fixed resistor, a fixed inductance and a voltage variable capacitance which allowed the remote control of the magnitude of the impedances. In some environments, such as mounting over an ideal ground plane, these two impedances 27 and 41 are matched, and in particular environments where the matching difficulties become substantial, it is contemplated that more sophisticated switching circuitry may be employed in order to use but a single impedance which is alternately connected to the antenna ends and 39. In other situations the two impedances may be made intentionally unequal to compensate for the particular antenna environment and/or to sharpen the direction finding resolution.

Turning now to FIG. 2, several of the wave forms which may occur in the circuitry of FIG. 1 are illustrated. Wave form (a) illustrates the square wave output of the square wave generator 53 and is illustrated primarily as a reference for the remaining wave forms which illustrate the switched carrier wave at (b) prior to detection for an assumed incoming signal along vector C and the output of the detector circuit 49 at (c) for incoming signals along the vectors A, B, C and D, respectively, of FIG. 3. As noted earlier, a signal along the vector A of FIG. 3 will yield maximum energization of the receiver 63 of FIG. 5 which is equivalent to yielding maximum energization of the receiver of FIG. 1 during the positive halves of the square wave output. This is illustrated as wave form (cA). The wave forms (b) through (cD) show the carrier or a part thereof primarily for illustrational purposes, and the detector 49 may be any type receiving device and may include a filter to elminate this carrier. In practice, the carrier frequency would be much higher than that illustrated since in a preferred embodiment the square wave frequency was 100 cycles per second and the carrier frequency involved was up in terms of megacycles.

Wave form (b) is merely an illustration of the contin uous wave carrier, the direction of which one might be using the antenna of the present invention to find since it like wave form (cC) assumes an incoming signal along vector C. However, wave form (b) does illustrate two discontinuities per cycle of the square wave generator 53, and these discontinuities which are a 180 phase reversal are the result of the switching circuitry which in effect reverses the directivity pattern of the antenna 11. A comparison of wave forms (c8) and (cD) which are the resulting detector output signals for incoming signals along the vectors B and D, respectively, of FIG. 3 shows that these two wave forms are identical except for a 180 phase reversal. To determine whether the signal corresponds to vector B or vector D, one need only compare the phase of the signal in question with the phase of the square wave generator. This is accomplished by the synchronous comparator 51 of FIG. 1. The dc component may be removed from any of the waves (cA) through (CD) and the magnitude or magnitude and phase of the remaining signal indicated on, for example. a zero center meter. It should be clear from the wave forms of FIG. 2 that wave form (cC) would yield a zero indication on such a meter indicating that the antenna, if it has symmetrical patterns, is physically oriented away from the direction of the incoming signal. Obviously, if the two lobes of FIG. 3 are not symmetrical, this angle may be other than 90. Wave form (cA) would yield a maximum meter indication, and the wave forms (cB) and (cD) would yield meter readings of equal magnitude; however, by incorporating the appropriate phase indication as a positive or negative output from the comparator 511, this meter may be made to appropriately deflect to the left or to the right depending upon the phase of the signals involved.

Turning now to FIG. 4, the improvement in bearing resolution attainable by terminating the antenna system of the present invention in an impedance other than the characteristic impedance of the antenna is illustrated. Assuming symmetrical lobes, a line perpendicular to the physical antenna (in the direction of vector C of FIG. 3) is referred to as the boresight of the antenna of the present preferred embodiment. and the abscissa of FIG. 4 is calibrated in degrees away from this boresight along which a signal may be incoming. The ordinate or vertical axis of the graph of FIG. 4 is the difference in db between the two detected signals or lobe responses of FIG. 3. To illustrate the resolution improvement in a readily attainable practical example, assume that the db difference is 1 db. This would correspond to a situation where, for example, wave form (CH) of FIG. 2 had its higher level of modulation about 1.25 times the power of the lower level of modulation, and such a difference would be easily detectable and indicatable by the indicator 55 of FIG. 1. With a matched termination antenna, the resolution attainable at this level would be about 7, whereas with a properly mismatched termination, this angle of resolution is easily reduced to about 2.

FIGS. 6 and 7 are analogous respectively to FIGS. 3 and 4 and illustrate the response characteristics for several different temiination impedances. In the specific example of FIGS. 6 and 7, the antenna length was slightly more than half of a quarter wave length, namely, the ratio of the length of the antenna to the wave length was 0.129. The terminating impedance used was a parallel inductance resistance combination with a reactive impedance equal to four-tenths of the characteristic impedance of the antenna. Under these circumstances, numerous curves are depicted for the loading resistance R,, represented as a decimal fraction of the antenna characteristic impedance 2,. These curves illustrate for a simple case how the directivity pattern and the angular resolution attainable may be varied for various terminating impedances.

A consideration of FIG. 6 will yield considerable insight into the benefits to be derived from mismatching the nonresonant antenna and its terminating impedance. Signals incoming along the boresight, that is along the 0 axis, are exemplary of the ones of interest in a direction finding situation. The 0 axis of course may not correspond to the boresight where the antenna system is not symmetrical. Note that for the curve labeled R =0.95Z,,. The magnitude of signals incoming along vectors displaced plus or minus 10 from the 0 axis are not substantially different from the magnitude of signals incoming along the 0 axis whereas for the curve labeled R, =.1Z,, these same magnitudes are subsimilar terminating impedances each of which is different from the characteristic impedance of the antenna to thereby provide a greater slope in the antenna directivity pattern near the boresight of the antenna.

12. The method of claim 11 wherein each said impedance is varied to improve the resolution capabilities of said system. 

1. A direction finder antenna system having controllable directional characteristics comprising: an antenna having a characteristic impedance and having first and second ends; a load adapted to function as an antenna terminating impedance and having an impedance different from the said characteristic impedance, the antenna system having different directional characteristics for different impedance values of said load; antenna utilization means; and means connecting said antenna utilization means to one of the first and second ends of the antenna and said load to the other of the first and second ends of the antenna.
 2. The antenna system of claim 1 further comprising a second load adapted to function as an antenna terminating impedance and having an impedance different from the said characteristic impedance, the antenna system having different directional characteristics for different Values of said load, and means for periodically interchanging the ends of said antenna to which said utilization means is connected and for connecting one of said loads to the end of said antenna opposite the utilization means.
 3. The antenna system of claim 2 further comprising means for independently varying the impedance of each said load to vary the directional characteristics of the antenna system.
 4. An antenna system comprising: an antenna having a characteristic impedance and having first and second ends; antenna utilization means; first and second loads each having a variable impedance different from the said characteristic impedance to thereby permit the directional characteristics of the antenna to be varied; a square wave generator having a frequency substantially less than the frequency to be received by the antenna system; first, second, third and forth semi-conductor devices, the first and second semi-conductor devices being connected in series between the antenna ends and having their common point connected to said utilization means, the third and forth semi-conductor devices each connected to a respective one of the antenna ends and to a respective one of the first and second loads and adapted to be switchably biased by the said square wave generator one to a conducting state and the other to a non-conducting state whereby one said variable impedance means is coupled by way of the third semi-conductor device to one of said antenna ends and the utilization means is coupled by way of the second semi-conductor device to the other of said antenna ends when said second and third semi-conductors are biased to their conducting state by the square wave generator and the other of said variable impedance means is coupled to the other antenna end by way of the forth semi-conductor device and the utilization means coupled by way of the first semi-conductor device to said one antenna end when the first and forth semi-conductor devices are biased to their conducting state by the square wave generator.
 5. A symmetrically switchable variable impedance antenna loading circuit comprising: a series circuit of four diodes adjacent ones of said diodes being coupled anode to cathode; an antenna having a first end connected to a first junction between the first and second diodes and a second end connected to a second junction between the third and fourth diodes; at least two variable impedance means, one coupled between the cathode of the first diode and a point of reference potential and the other coupled between the anode of the fourth diode and said point of reference potential; means coupling a third junction between the second and third diodes to an antenna utilization device; and means for periodically varying the potential at said first and second junctions.
 6. The antenna loading circuit of claim 5 wherein said means for periodically varying comprises a square wave oscillator having an output frequency substantially less than the frequency to be received by said antenna; and radio frequency choke means coupling said square wave oscillator to said first and second junctions, the impedance of said radio frequency choke means being relatively high at the frequencies to be received by said antenna and relatively low at said square wave oscillator frequency.
 7. The method of loading a nonresonant antenna system for at least one of; compensating for irregularities introduced by mounting said antenna on for example an irregular ground plane, and varying the directional characteristics of said antenna system, said method comprising the steps of: connecting one end of an antenna to a utilization device; and connecting the other end of said antenna to a load having an impedance different from the characteristic impedance of said antenna.
 8. The method of claim 7 further comprising the step of varying the impedance of said load.
 9. The method of claim 8 further comprising the step of periodically inTerchanging the ends of said antenna to which said utilization device and the load are connected.
 10. The method of claim 7 further comprising the step of periodically interchanging the ends of said antenna to which the load and said utilization device are connected.
 11. The method of loading a non-resonant antenna system to improve its direction finding capabilities comprising the steps of: periodically connecting one end of the antenna to a utilization device and then reversing the end of the antenna to which said utilization device is connected; and periodically connecting the end of the antenna remote from said utilization device to one of two dissimilar terminating impedances each of which is different from the characteristic impedance of the antenna to thereby provide a greater slope in the antenna directivity pattern near the boresight of the antenna.
 12. The method of claim 11 wherein each said impedance is varied to improve the resolution capabilities of said system. 